The present invention relates to an optical limiter for use in optical systems. In particular, the optical limiter limits radiation travelling along a path so as to protect an optical element such as a photodetector or the human eye from high intensity radiation which could damage that optical element.
Optical limiting or protective devices for optical systems have been of interest for a number of years. The major use of an optical limiter is to protect a photodetective device from high intensity radiation which could damage the device. For example, infrared detectors can be susceptible to damage from high intensity infrared radiation from a carbon dioxide laser. Similarly, both semiconductor photodetectors and the human eye are susceptible to damage caused by radiation from intense sources such as a nuclear explosion or the sun.
In many cases, it is not practical to totally filter out the particular wavelength or wavelengths of the high intensity source. Often these are the very wavelengths to which the photodetector is required to be responsive. For example, an infrared detector designed to operate in the 8 to 14 micron wavelength region cannot have these wavelengths completely filtered out.
The purpose of the optical limiter, therefore, is to limit the amount of radiation reaching the detector to a level which will not damage the detector. Many devices and systems have been suggested as optical limiters. U.S. Pat. Nos. 3,152,215 by Barstow et al and 3,714,430 by Finvold et al are examples of two types of optical limiters.
Many of these devices operate on the basis of an effect which shifts with temperature. The Finvold et al patent is an example of this type of device. Finvold et al describes the use of a variable density filter and a spectral bandpass filter or long wavelength rejection filter. The spectral filter is a thin slice of semiconductor direct bandgap material which has a very steep absorbing edge adjacent its inherent emitting line so that it passes the desired emission line at ordinary temperatures and attenuates the emission line at elevated temperatures.
A number of other devices, such as modulators, have been suggested which use temperature-dependent effects to provide modulation or variable attenuation of radiation. Examples of these types of devices are described in U.S. Pat. No. 2,964,619 by Hahn et al, 2,982,856 by Camp, and 3,025,763 by Schwartz et al.
Despite the interest and efforts in the field of optical limiters, further improvements are still desirable. For example, an optical limiter which uses a small number of relatively simple and inexpensive components is desirable.